Conventionally, there is proposed a speech recognition apparatus that uses tree structured dictionary data for speech recognition (e.g., see patent document 1). The tree structured dictionary data contains each of multiple words as a node in the tree structure. The speech recognition apparatus determines to what degree an input speech matches a string of words (hereafter referred to as a word string) along a path from a root of the tree structure to a leaf node. To perform the determination, the speech recognition apparatus sequentially compares the input speech with an acoustic model corresponding to the word string in accordance with a sequence of the word string.
In addition, there is proposed a speech recognition apparatus that recognizes an input speech in the order reverse to another order used to input the speech. The speech recognition apparatus performs the reverse recognition using backward tree structured dictionary data in which words are arranged in the order of utterance from a leaf node to the root according to the tree structure.
For example, the speech recognition apparatus described in patent document 2 can recognize an unpunctuated input speech representing a street address uttered in the order of street, city, and state according to the system in Europe or the United States. The speech recognition apparatus sorts a waveform or a feature quantity of the input speech in reverse order of the utterance. The speech recognition apparatus determines to what degree the sorted backward speech matches a word string arranged from the root to a leaf node in the backward tree structure. To do this, the speech recognition apparatus sequentially compares an acoustic model corresponding to the word string with the backward speech in accordance with a sequence of the word string.                Patent Document 1: JP-11-231894 A        Patent Document 2: JP-2003-114696 A        
However, the above-mentioned speech recognition apparatus technology uses the backward tree structured dictionary data and cannot complete the recognition until a user utters the whole of a word string arranged from the leaf node to the root of the tree structure.